1. Field of the Invention
The present invention relates to a power train such as a propeller shaft of a vehicle. Especially, the present invention relates to a shock-absorbing construction of a constant velocity joint used for the power train of a three-joint type.
2. Description of the Related Art
Japanese Utility Model First Publication No. Heisei 2 (1990)-117222 discloses a power train applied to a propeller shaft of a vehicle.
The power train according to Japanese Utility Model First Publication No. Heisei 2 (1990)-117222 is applied to the propeller shaft of a three-joint type. The power train has a tubular shaft body separated into two, and is used for a four-wheel drive (4WD) vehicle or a rear-wheel drive vehicle.
By way of an intermediary bearing, a stub shaft connects to a first shaft which connects to a transmission. In addition, the tubular shaft body connects to the stub shaft by way of a constant velocity joint.
The tubular shaft body has a first end section constituting an outer race of the constant velocity joint. The first end section of the tubular shaft body is partly tapered. The constant velocity joint has an inner race, a cage, and a ball which are smaller in diameter than an inner diameter of the tubular shaft body. With the above construction, the inner race and the like of the constant velocity joint can enter the tubular shaft body following a backward movement of an engine and/or a transmission in a vehicular collision, to thereby absorb a shock which may be caused in the vehicular collision.
The tubular shaft body housing therein the constant velocity joint can be separated into two sections. More specifically, one is a tubular holder disposed on the first end side and constituting the outer race, and the other is a tubular shaft section disposed on a second end side opposite to the first end side. The thus separated two members, that is, the tubular holder and the tubular shaft section are coupled through a friction welding. Instead of an arc welding, the friction welding is to be used for the following reasons:                Heat caused by the arc welding may be responsible for a thermal effect on each of members constituting the constant velocity joint that is supposed to have high dimensional accuracy.        Contrary to the arc welding, the friction welding can cause the heat to weldment only (local heat), thus reducing the thermal effect on each of the members constituting the constant velocity joint. In addition, the friction welding can increase vibration accuracy during rotation of the propeller shaft.        
Coupling the holder with the tubular shaft section through the friction welding, however, may cause an inner curl and an outer curl to the weldment. The outer curl can be deleted by cutting, thus finishing a smooth surface of the weldment. On the other hand, cutting the inner curl is of difficulty. In addition, cutting the inner curl may excessively reduce the friction welding strength. In sum, the inner curl has no choice but to remain left.
In the vehicular collision and the like, the backward movement of the engine and/or the transmission may cause the stub shaft of the propeller shaft to slide each of the members {constituting the constant velocity joint} in the tubular shaft body. Hereinabove, the members of the constant velocity joint include the ball, the inner race, and a seal plate incorporating a lubricant grease. An outer periphery of the seal plate thus slid in the tubular shaft body may abut on an edge of the inner curl, thereby preventing further sliding movement.
Prevention of the further sliding movement of the members of the constant velocity joint may be responsible for an insufficient shock-absorption, resulting in a greater shock to a seat occupant.